In an image forming system, an optional sheet post-process apparatus can be connected to an image forming apparatus such as a multifunction peripheral. Recently, a sheet post-process apparatus is proposed which has a function that aligns ends of a stack of sheets printed by the multifunction peripheral are aligned in length (longitudinal) and width (lateral) directions, and performs saddle stitch binding of the stack of sheets to obtain a booklet.
As the sheet post-process apparatus, US Patent Application Publication No. 2004/0254054A1 discloses a sheet folding device that pushes out a folding plate in the direction perpendicular to a vertical sheet conveying path to insert the sheet or sheet stack between a pair of folding rollers and fold the sheet or sheet stack nipped and fed by the folding rollers.
In the paper folding device, the rear ends (lower ends) of the sheets stacked between stack conveying guide plates disposed along a sheet conveying path are supported by a movable rear end fence and elevated along the sheet conveying path.
Since the sheet having no rigidity (firmness) is buckled on the movable rear end fence, accurate positioning of the sheet stack can not be attained by the elevation control of the movable rear end fence. Also with respect to a curled sheet, accurate positioning of the sheet stack can not be attained by the elevation control of the movable rear end fence. In addition, when a sheet is caught by the sheet conveying guide plate, it becomes difficult to align the lower ends of the sheets.